We asked if someone could help us get a logo for Gjallar based on what Gjallar means. The winner came from Elfi Heck:
Original in SVG: gjallar.svg
The association is of course the "warning horn" and not the "drinking horn". I have also asked Elfi for a variation with the name "Gjallar" or "Gjallar!" somehow added (along the upper curve or perhaps being screamed out of the horn or something).